Electronic device for sound reproduction of musical instruments



Aug. 28, 1951 F. T. COOPER 2,565,911

ELECTRONIC DEVICE FOR SOUND REPRODUCTION OF MUSICAL INSTRUMENTS Filed Dec. 10, 1,948 2 Sheets-Sheet 1 Aug. 28, 1951 F. T. COOPER 2,565,911

ELECTRONIC DEVICE FOR SOUND REPRODUCTION OF MUSICAL INSTRUMENTS Filed Dec. 10, 1948 2 Sheets-Sheet 2 FIG. 4.

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Patented Aug. 28, 1951 ELECTRONIC DEVICE FOR SOUND REPRO- DUCTION OF MUSICAL INSTRUMENTS Frank T. Cooper, Santa Cruz, Bombay, India Application December 10, 1948, Serial No. 64,686 In India January 28, 1948 This invention relates to musical instruments, and more particularly to an electronic pickup device for use with musical instruments of the stringed type, such as pianos, guitars, and the an improved pickup device for use with a stringed instrument in conjunction with an electronic amplifier, said pickup device being very simple in construction, compact in size, so that it is easy to install where space is restricted, and which is very durable, so that its sensitivity remains substantially constant over long periods of time, and regardless of whether or not the instrument is used during said time periods.

Further objects and advantagesof the invention will become apparent from the following description and claims, and from the accompanying drawings, wherein:

Figure l is a front elevational view, partly broken away, of a conventional piano equipped with an electronic sound pickup device constructed in accordance with and installed in accordance with the present invention.

Figure 2 is an enlarged cross-sectional detail view taken on line 2-2 of Figure 1, showing one method of installation of the pickup device.

Figure 3 is a view similar to Figure 2 but illustrating the method of installation of the pickup device employed with a piano differing somewhat in construction from the piano of Figures 1 and 2'.

Figure 4 is an enlarged longitudinal vertical cross-sectional view taken on line 44 of Figure 2, showing the internal structural details of one sectional unit of the pickup device.

Figure 5 is a top plan view, partly broken away, of the pickup device sectional unit shown in Figure 4.

Figure 6 is an exploded side elevational view of the core portion of the pickup device sectional unit of Figures 4 and 5, showing the various com ponents of said core portion.

Figure '7 is a schematic wiring diagram showing the manner in which the pickup device is connected to an electronic amplifier.

Referring to the drawings, and more particularly to Figures 1 and 2, H designates a conven- 3 Claims. (o1. s4 1.14)

tional piano provided with the usual arrangement of strings l2 and provided with an action including the hammers I3 and the dampers [4 arranged above the hammers. In accordance with the present invention a wooden support bar [5 is secured to the frame members l6 of the piano, said support bar 15 being closely adjacent to and transverse to the strings l2. Secured to the support bar l5 by means of bolts l7, IT in each of the respective string sections defined by the spaced frame members I6 is a pickup element l8, said pickup element extending parallel to the support bar l5, being held closely adjacent the strings l2, and being substantially coextensive with the length of the associated string section.

As shown in Figure 3, if the piano action is of a design wherein the dampers, shown at M are located below the hammers, shown at 13', the support bar I 5 is fastened to the piano frame above the hammers and the pickup elements I8 are suspended from the bar l5 by means of the bolts H. In the design shown in Figures 1 and 2, the support bar [5 is fastened to the piano frame inwardly adjacent and below the hammers, and the pickup elements l8 are supported above bar l5 by means of said bolts, as shown in detail in Figure 4. r

As shown in Figures 4 to 6, each pickup element 18 comprises a pair of parallel spaced flat iron bars I 9 and '20 between which are secured a plurality of spaced permanent magnets 2l, said magnets having their north poles in contact with bar l9 and their south poles in contact with bar 20. The bolts I! pass through the end portions of the end magnets 2|, as shown in Figure 4. Shorter bolts 2'! are employed to secure the magnets to the bars inwardly of the end bolts IT.

The magnets 2| are polarized longitudinally as well'as vertically and are positioned with 1ongitudinal ends of like polarity adjacent each other as shown in Figure 6, wherein N and S designate the vertical poles of the magnets and E and W designate the longitudinal poles of the magnets. The outer ends of the end magnets are of like polarity, for example, both being east poles, as indicated in Figure 6. The respective bolts l1 and 21 are of non-magnetic material, such as brass.

By the arrangement above described, the permanency of the composite magnet core is better assured, inasmuch as, if the magnets 2| were mounted with their E and W poles adjacent each other there would be a tendency for the magnets to dissipate their longitudinal magnetic forces through one another and through the adjacent bridging portions of the pole bars [9 and 20. Also by this arrangement the flux density in the air space between the marginal portions of the pole bars l9 and 2B is increased. As shown in Figures 5 and 6, the bars l9 and 28 project substantially beyond the side planes of the magnets H and also project beyond the end planes of the magnets at the ends of the pickup element, whereby the composite core is defined. Wound: on this core is a coil 22 which is confined within the space between the bars l9 and 2t.

Since the iron bar i9 is in contact with all the north pole faces of the magnets 2!, bar l9 becomes the north pole of the composite magnet defined by the above described assembly, and similarly, bar becomes the south pole thereof. The magnetic flux threads through the turns of the coil 22, and any fluctuation of said flux induces a corresponding voltage in said coil.

As shown in Figure 4, the pickup elements it are supported on the support bar it by the elongated end bolts I'Lsaid bolts being provided with suitable nuts 23 for holding the pickup elements in parallel spaced relation to the support bar. The pickup elements'ls' are therefore held with the longitudinal edges'of their pole bars 59 and 20 closely adjacent to and transverse to the piano strings I2, with the magnetic flux between the polebars threading around the piano strings with substantial density. Vibration of. a string therefore produces a corresponding vibratory fluctuation of the magnetic flux, which, induces a corresponding fluctuating voltage in. the associated coil 22.

The coils 22 may consist of any desired number of turns, but preferably consistof a small number of turns of heavy wire to increase the ruggedness of the unit. As shown in Figure the respective coils 22 of the pickup elements are connected in series, and the resultant series pickup circuit is connected to the low-impedance primary of an output transformer 24 whose secondary is of high impedance and is connected to the input terminals of a conventional electronic amplifier 25. The amplifier output is connected to a pair of line wires 26, 26 across which one. or more loud speakers may be connected. Amplifier may be mounted within the piano or may be external thereto, and is provided with suitable gain control means to regulate the degree of amplification thereof.

Since each string I2 of the piano acts to produce a flux variation when vibrated, the strings act as individual voltage generators with respect to the coils 22 and the resulting voltages are amplified in a desired degree by the amplifier 25. Of course, the string struck can only affect a section of the coil 22 adjacent thereto but that is sufficient to generate a sufiicient signal voltage in the entire coil to feed the amplifier. Other strings struck will affect different parts of the coil 22 and spread through the coil. For example, assume that a chord facing a single coil is struck. Several operative voltages are generated in this single coil but these several voltages are of different frequencies and they will be reproduced consequently as a chord through the amplifier, the volume of sound being greater than that produced by a single note. The amplifier setting is, however, kept at a level where, when the maximumnumber of strings which can possibly be struck are thus struck, there will be no audible distortion. The greater volume of scribed, with. a resultant increase in volume and a modification of tone quality which may be regulated by suitable adjustment of amplifier 25. By the use of suitable output filter devices in the amplifier, well known per se, the sound output of the system may be regulated so as to produce a predominance of either bass, treble, or intermediate tones, whereby numerous interesting tonal effects may be obtained.

In view of the thin elongated shape of the pickup elements [8, said elements may be readily mounted in a restricted space and therefore can be installed in practically all designs of pianos without modification of the pianos. While disclosed specifically herein as being employed with pianos, the device may be installed on any stringed instrument employing strings having magnetic permeability, such as on steel guitars, and the like.

The magnets 2| are preferably of the highdensity type, such as Alnico, or the like, having high stability. Since a plurality of magnets 21 are employed in each pickup element, deterioration of one. of the magnets. will not. substantially impair the sensitivity of the pickup element, and the pickup device will remain operative over long periods of time.

.While a specific embodiment of an. electronic sound reproducing device for. stringed musical instruments has been disclosed inthe foregoing description, it will be understood that various modifications within the spirit of theinvention may occur to those skilled in the art. Therefore it is intended that no limitations be placed on the invention except asv defined by the scope of the appended claims.

What is claimed is:

1. In combination with a stringed instrument. a pickup element mounted transverse to the strings of the instrument and adjacent to said strings, said element comprising a pair of parallel elongated fiat bars of magnetically permeable material having opposing, longitudinal margins, a plurality of permanent magnets secured between the bars at longitudinally spaced inter vals with their north pole faces. in contact with one of the bars and their south pole faces in contact with the other bar, said magnets being ar-- ranged with adjacent longitudinal ends'of like polarity, and a coil wound on and surrounding the magnets and extending longitudinally between the bars with its turns positioned in the spaces between the respective opposing longitudinal. margins and the ends of the bars.

2. In combination with a piano, a pickup element mounted on the piano frame transverse to the strings of the piano and closely adjacent to said strings, said element comprising a pair of parallel flat elongated iron bars having opposing longitudinal margins, a plurality of permanent magnets disposed between the bars at longitudinally spaced intervals and having adjacent longitudinal ends of like polarity, the magnets having their north pole faces in contact with one of the bars and their south pole faces in contact with the other bar, transverse fasteners securing said magnets between the bars, the longitudinal margins of the bars extending substantially beyond the side faces of the magnets, and a coil Wound on and surrounding the magnets and extending longitudinally between the bars with its turns positioned in the spaces between the respective opposing longitudinal margins of the bars, the planes of the bars being substantially normal to the plane of the piano strings, whereby a substantial density of magnetic flux is present around the piano strings.

3. In combination with a piano, electronic pick-up means mounted on the piano and including a plurality of pick-up elements disposed transversely to the strings of the piano and adjacent thereto, said pick-up elements being longitudinally spaced and Wired in series with an amplifier, each of said elements comprising a pair of parallel flat iron bars having opposing longitudinal margins, a plurality of permanent magnets secured between the bars at longitudinally spaced intervals with their north pole faces in contact with one of the bars and their south pole faces in contact with the other bar, the longitudinal margins of the bars extending substantially beyond the side faces of the magnets, and a coil wound on and encompassing the magnets and extending longitudinally between the respective opposing longitudinal margins of the bars, the planes of the bars being substantially at right angles to the plane of the piano strings, whereby substantial density of magnetic flux is present around the piano strings.

FRANK T. COOPER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,814,798 Guedon July 14, 1931 1,870,576 Knoblaugh Aug. 9, 1932 2,170,694 Perry Aug. 22, 1939 2,258,990 Lundie Oct. '14, 1941 2,263,973 OBrien Nov. 25, 1941 2,455,567 De Armond Dec. 7, 1948 

